1. Field of the Invention
The present invention concerns a charge-coupled device with lowering of transfer potential at output, as well as a method for the fabrication of this device.
It can be applied to the making of memories, shift registers, delay lines, television cameras etc.
2. Description of the Prior Art
Charge-coupled devices are semiconductor-based systems wherein the charges are stored in potential wells. These charges are created at the surface of a semiconductor layer generally formed on a substrate, or at the interface between the semiconductor layer and an insulating surface layer (in a so-called buried-channel device). The charges are transferred from an upstream position towards a downstream position in successively occupying, through transfer potentials, all the potential wells created between these two positions. In general, and to put it in a very simplified way, these devices comprise a semiconductor substrate with a certain type of doping, a layer of an insulating material, and a set of electrodes on this insulating layer carried to appropriate potentials. This insulating material may be an oxide. The charges shifted in devices of this type are electrons.
Another known type of charge-coupled device, which is more efficient than the above ones, is shown schematically, in a sectional view, in FIG. 1. This device comprises, on the semiconductor substrate with a first type of doping (P type doping for example), a semiconducting layer 2 with a second type of doping (N type). An insulating layer 3 (an oxide for example) coats the semiconducting layer 2. This device also has at least one first pair of electrodes 4, 5 and at least one second pair of electrodes 6, 7, along an axis oriented in a direction X (FIG. 3), defining a direction of flow of the charges, said pairs of electrodes being between between an upstream position where the charges are created, and a downstream position. This figure also shows other pairs of electrodes 8, 9 and 10, 11, which are identical to the preceding pairs of electrodes. The charges are created on the upstream (of the electrode 4) position side by prior art means which shall therefore not be shown.
Each pair of electrodes such as 4, 5, has, in the direction of flow X, a transfer electrode 4, and a charge storage electrode 5. These electrodes are in contact with the insulating layer 3, and the storage electrodes such as the electrode 5 are coated with an electrically insulating coat 12 (an oxide for example).
This known prior art device also has means which, combined with electrical potentials applied to the electrodes, enable the creation of potential wells of equal depths, beneath and facing the storage electrodes 5, 7, 9, and identical, disymmetrical, transfer potentials beneath and facing the transfer electrodes 4, 6, 8 . . . and the storage electrodes 5, 7, 9 . . . . These transfer potentials and these potential wells make it possible, in a known way, to cause the charges to flow in the chosen direction X. They appear at the interface between the semiconductor layer 2 and the insulating layer 3. In known way, the means used to create these potential wells and these transfer potentials comprise zones with a third type of doping (N.sup.- in the example considered), which are made in the second type semiconducting layer 2, beneath and facing transfer electrodes 4, 6, 8 . . . etc. These means also have two electrical voltage sources V1, V2. The voltage source V1 is connected to the pairs of electrodes 4-12, 8-9, while the voltage source V2 is connected to the pairs of electroodes 6-7, 10-11.
The voltages V1, V2 vary cyclically and in phase opposition between identical values, as shown in FIG. 2 (for example between the value 0 and a positive value).
In this type of device, since the potentials applied to the successive pairs of electrodes have the same extreme values, and the semiconducting layer 2 with a second type of doping (N for example) have identical zones 15 with a third type of doping (N.sup.- for example), the potential wells beneath and facing the storage electrodes have the same depth throughout the length travelled by the charges in the device. The transfer potentials also have identical variations in levels throughout the length travelled.
FIG. 3 gives a better understanding of the operation of this prior art device. This figure shows the profile of the potential V, along the length of the device of FIG. 1, in the direction of flow X. This potential is the one that appears at the interface between the semiconducting layer 2 and the insulating layer 3, when the voltages V1, V2, in phase opposition are applied to pairs of electrodes as indicated above. In this figure, the curve 01 corresponds to the surface potential appearing beneath the electrodes when the voltage V1 is applied to the corresponding electrodes, while the curve 02 represents the surface potential when the voltage V2 is applied to the corresponding electrodes. This surface potential depends on the density of the dopant at all points of the perpendicular to the point of the surface considered. It also depends on the potentials applied to the electrodes. The preence of the layer with a second type of doping (N in the example considered), beneath the electrodes 5, 7, 9, causes an increase in the depth of the potential well in this region. On the contrary, beneath the transfer electrodes, for which there is a zone with a third type of doping (N.sup.- in the example considered) the potential well is not as deep. The result thereof is that, by causing variation in the potentials applied to the electrodes, there is created, in the interval between these electrodes, an asymmetric potential well which is deeper downstream than upstream. When the voltage V1 or V2, applied to the storage electrodes 5, 7, 9, is at a maximum, the depth of the potential wells PU1, PU2 . . . , created beneath the electrodes, is itself at a maximum, and is equal VP for example. The electrons are then stored in these potential wells as shown in FIG. 3. On the contrary, when the voltage applied to the corresponding storage and transfer electrode diminishes, the depth of the potential well diminishes and the electrons which were stored in the potential well PU1 are transferred to the potential well PU2 beneath the following storage electrodes. The potential bottom flat levels, such as VP1, VP2, favor this transfer.
It may be sometimes useful, especially (as shall be seen further below in greater detail) in an charge reading device located downstream, at the end or output of the device, to lower the values of the transfer potentials at this downstream end of the device.
At present, there is no device that provides a simple way to reduce the values of the bottom flat levels of the transfer potentials at a downstream end of a charge-coupled device. The result thereof is that the read amplifiers, which are connected to the downstream ends of known charge-coupled devices of the above-described type, have a supply voltage which is sometimes considerably higher than the electrical supply voltages of the assemblies in which these charge-coupled devices are used.